Multi-wavelength all-optical regenerators ( mars)

ABSTRACT

In an optical regenerator for use in an optical transmission system, an optical input signal to the regenerator comprising N wavelength division multiplexed channel signals is received in a signal synchronizing arrangement which synchronizes the phase of all of the N channels, where N·2. An optical output signal from the signal synchronizing arrangement has N phase-synchronized wavelength division multiplexed channels and is provided as an optical input signal to a nonlinear optical loop mirror (NOLM) arrangement. The NOLM has a high nonlinearity fiber loop that propagates equal parts of the optical input signal from the signal synchronizing arrangement in both a clockwise (CW) and a counterclockwise (CCW) direction. An optical clock signal from a laser having a phase that matches the phase of the CW and CCW propagating signals is multiplexed with the CW signal to provide a 180 degree phase shift between the CW and CCW propagating signals while propagating in the optical fiber loop. The NOLM retimes and reshapes the optical pulses of the optical synchronized N wavelength division multiplexed channels, and generates a corresponding optical NOLM output signal to an Erbium-doped fiber amplifier (EDFA). The EDFA amplifies the NOLM output signal, and generates an amplified and phase-synchronized optical N wavelength division multiplexed channel output signal that is transmitted from the optical regenerator.

FIELD OF THE INVENTION

[0001] The present invention relates to multi-wavelength, all-optical,regenerators for use in high speed optical communication systems, and toa method for regenerating an optical N wavelength division multiplexedchannel signal received by a regenerator.

BACKGROUND OF THE INVENTION

[0002] Conventional regenerators that are used in optical networksconvert received optical signals to electronic signals and then back tooptical signals for transmission to a next regenerator or terminalstation. The problem with such conventional type of regenerators is thatthey have a bandwidth which is limited to 10 gigahertz and each channelhas to be processed on a channel-by-channel basis. As point-to-pointtransmission speeds in communication systems approach 10 gigabits/second(e.g., in OC-192 systems) and beyond, it is highly desirable to useall-optical regenerators to extend transmission distances betweenregenerators (repeaters) instead of using the conventional type ofregenerators that convert received optical signals to electronic signalsand then back to optical signals. From a network point of view, theconventional electronic type of regenerators become bottlenecks for highcapacity optical networks. In contrast, all-optical regenerators canprovide a larger bandwidth and have many advantages when consideringnetwork scalability, management, and capacity. Therefore, all-opticalregenerators will be key components for future all-optical communicationnetworks.

[0003] It is desirable to provide a regenerator system and a methodwhich simultaneously reamplifies, retimes, and reshapes receivedmultiple wavelength division multiplexed channels entirely in theoptical domain before retransmission along a next section of an opticalcommunication system.

SUMMARY OF THE INVENTION

[0004] The present invention is directed to multi-wavelength all-opticalregenerators, where each regenerator reamplifies, retimes, and reshapesmultiple wavelength channels in a received N wavelength divisionmultiplexed optical channel signal entirely in the optical domain.

[0005] Viewed from one aspect, the present invention is directed to anoptical regenerator for use in an optical transmission system comprisingan optical N wavelength division multiplexed channel signalsynchronizing arrangement, an optical pulse retiming and reshapingarrangement comprising a nonlinear optical loop mirror (NOLM)arrangement, and an Erbium-doped fiber amplifier (EDFA). The optical Nwavelength division multiplexed channel signal synchronizing arrangementis responsive to the reception of an optical input signal to theregenerator comprising N wavelength division multiplexed channel signalsfor synchronizing the phase of all of the N channel signals, and forgenerating an optical output signal comprising synchronized N wavelengthdivision multiplexed channel signals, where N·2. The nonlinear opticalloop mirror (NOLM) arrangement in the optical pulse retiming andreshaping arrangement is responsive to the optical output signal fromthe signal synchronizing arrangement for retiming and reshaping opticalpulses of the optical synchronized N wavelength division multiplexedchannel signals, and for generating a corresponding optical NOLM outputsignal. The Erbium-doped fiber amplifier (EDFA) amplifies the opticalNOLM output signal, and generates an amplified and phase-synchronizedoptical N wavelength division multiplexed channel output signal fortransmission as an output signal from the optical regenerator.

[0006] Viewed from another aspect, the present invention is directed toan optical regenerator for use in an optical transmission systemcomprising an optical wavelength division demultiplexer (WDD), aplurality of N−1 selectively changeable optical delays, an opticalwavelength division multiplexer (WDM), an optical pulse retiming andreshaping arrangement comprising a nonlinear optical loop mirror (NOLM)arrangement, and an Erbium-doped fiber amplifier (EDFA), where N·2. TheWDD is responsive to an optical input signal to the regeneratorcomprising N wavelength division multiplexed channels for directing theN channels onto separate N output paths. Each of the plurality of N−1selectively changeable optical delays is coupled in a separatepredetermined one of N−1 output paths of the WDD. Still further, eachselectively changeable optical delay is responsive to a delay controlsignal for selectively adjusting the phase of the channel signalpropagating in an associated one of an N−1 output path so that all N−1optical channel signals are synchronized in phase with an Nth opticalchannel signal. The WDM multiplexes the Nth optical channel signal andthe N−1 optical channel signals from the plurality of N−1 optical delaysinto a synchronized N wavelength division multiplexed channel opticaloutput signal. The phase controller is responsive to a portion of theoutput signal from the WDM for detecting a phase difference between theNth channel signal and each of the remaining N−1 delayed channelsignals. In turn, the phase controller generates a separate delaycontrol signal to each of the optical delays for synchronizing the phaseof each of the N−1 delayed optical channel signals to the phase of theNth optical channel signal. The optical pulse retiming and reshapingarrangement comprising a nonlinear optical loop mirror (NOLM)arrangement which is responsive to the optical output signal from theWDM for retiming and reshaping optical pulses of the opticalsynchronized N wavelength division multiplexed channel signals and forgenerating a corresponding optical NOLM output signal. The EDFAamplifies the NOLM output signal, and generates an amplified andphase-synchronized optical N wavelength division multiplexed channeloutput signal for transmission as an output signal from the opticalregenerator.

[0007] Viewed from still another aspect, the present invention isdirected to an optical regenerator for use in an optical transmissionsystem comprising an optical N wavelength division multiplexed channelsignal synchronizing arrangement, an Erbium-doped fiber amplifier(EDFA), and an optical pulse retiming and reshaping arrangementcomprising a nonlinear optical loop mirror (NOLM) arrangement, a clockrecovery circuit, and a laser. The optical N wavelength divisionmultiplexed channel signal synchronizing arrangement is responsive tothe reception of an optical input signal to the regenerator comprising Nwavelength division multiplexed channels for synchronizing the phase ofall of the N channel signals, and for generating an optical synchronizedN wavelength division multiplexed channel output signal, where N·2. Thenonlinear optical loop mirror (NOLM) arrangement is responsive to theoptical output signal from the signal synchronizing arrangement forretiming and reshaping optical pulses of the N channel signals, and forgenerating a corresponding optical NOLM output signal. The clockrecovery circuit is responsive to a predetermined one of the N channelsignals propagating in the optical synchronizing arrangement forgenerating a clock output signal having a frequency corresponding to thebit rate frequency of the predetermined one of the N channel signals.The clock recovery circuit is also responsive to an output signal fromthe optical regenerator comprising N phase-synchronized optical Nchannel signals for synchronizing the phase of the clock output signalto the phase of the N channel signals in the output signal from theoptical regenerator. The laser is responsive to the clock output signalfrom the clock recovery circuit for generating a corresponding opticalclock output signal which is coupled into the NOLM. The Erbium-dopedfiber amplifier (EDFA) is used to amplify the optical NOLM outputsignal, and generate an amplified and phase-synchronized optical Nwavelength division multiplexed channel output signal for transmissionas the output signal from the optical regenerator.

[0008] Viewed from still another aspect, the present invention isdirected to a method of regenerating a received optical N wavelengthdivision multiplexed channel signal in an optical regenerator of anoptical transmission system, where N·2, comprising the following steps.In a first step, the phase of all of the N channel signals in thereceived optical N wavelength division multiplexed channel signal areconcurrently and separately synchronized for generating an opticaloutput signal comprising N synchronized wavelength division multiplexedchannel signals. In a second step, the optical output signal from step(a) is split into two halves in an optical coupler so that a first halfpropagates in a first direction around an optical fiber loop wherein atleast a portion thereof comprises a high nonlinearity optical fibersection and a second half propagates in a second opposite directionaround the optical fiber loop. In a third step, an optical clock outputsignal from a laser is multiplexed with the first half signalpropagating in the first direction in the optical fiber loop in thesecond step in an optical wavelength division multiplexer. This shiftsthe phase of the first half signal in the optical fiber loop by apredetermined amount and causes the first and second halves of the Nchannel signals to be switched out of a second port of the opticalcoupler as a NOLM output signal when the two halves return to the entrypoint of the optical fiber loop. In a fourth step, the optical NOLMoutput signal from the third step is amplified in an Erbium-doped fiberamplifier (EDFA) for generating an amplified and phase-synchronizedoptical N wavelength division multiplexed channel output signal fortransmission as an output signal from the optical regenerator.

[0009] The invention will be better understood from the following moredetailed description taken with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWING

[0010]FIG. 1 shows a block diagram of a multi-wavelength, all-optical,regenerator system (MARS) in accordance with the present invention; and

[0011]FIG. 2 is a block diagram of a phase controller for use in themulti-wavelength, all-optical, regenerator system (MARS) of FIG. 1 inaccordance with the present invention.

DETAILED DESCRIPTION

[0012] Referring now to FIG. 1, there is shown a block diagram of amulti-wavelength, all-optical, regenerator system (MARS) 10 (shownwithin a dashed line rectangle) in accordance with the presentinvention. The MARS 10 is a photonic device that simultaneously performsthe functions of reamplification, retiming, and reshaping of Nwavelength division multiplexed channels in the optical domain withoutconverting the received signal format from an optical signal to anelectronic signal and back to an optical signal (a so-called o-e-oconversion) before retransmission along another section of an opticalcommunication system.

[0013] The MARS 10 comprises an optical wavelength divisiondemultiplexer (WDD) 20, first, second, and third optical taps 22, 32,and 44, respectively, a plurality of N−1 selectively changeable opticaldelays (DELAY) (of which only three delays 24, 25, and 26 are shown), anoptical wavelength division multiplexer (WDM) 30, an optical isolator34, a phase controller 40, an Erbium-doped fiber amplifier (EDFA) 42, aclock recovery circuit 50, a laser 52, and a nonlinear optical loopmirror (NOLM) 60 (shown within a dashed line rectangle). The combinationof the WDD 20, the first and second optical taps 22 and 32, theplurality of N−1 selectively changeable optical delays 24, 25, and 26,the WDM 30, and the phase controller 40 can be considered as a signalsynchronizing arrangement. similarly, the combination of the clockrecovery circuit 50, the laser 52, and the NOLM 60 can be considered asa pulse retiming and reshaping arrangement. The NOLM 60 comprises a50:50 coupler 70 that is coupled to both ends of a nonlinear opticalloop 71. The nonlinear optical loop 71 provides a serial coupling of aoptional first polarization controller (POLAR. CONT.) 72 (shown within adashed line rectangle), an optical wavelength division multiplexer (WDM)76, a loop of a dispersion-shifting high nonlinearity fiber 80, and afilter (FILT.) 82. The NOLM 60 further comprises a second optionalpolarization controller (POLAR. CONT.) 74 (shown within a dashed linerectangle) which couples an output of the laser 52 to an input of theWDM 76.

[0014] In the MARS 10, an optical input signal comprising N wavelengthdivision multiplexed channels is provided at an input 18 as an input tothe WDD 20, where N can comprise any integer ·2 such as 4, 8, 16, 32,etc. However, for purposes of explanation hereinafter, it is assumedthat N=4. The WDD 20 is a wavelength division demultiplexer whichdirects the N wavelength division multiplexed channels along N separateoutput paths. A first channel (CH.1) of the N wavelength divisionmultiplexed channels, which is hereinafter considered as a referencechannel, is directed to an input of the first optical tap 22. The secondchannel (CH.2), the third channel (CH.3), and the Nth channel (CH.N) ofthe N wavelength division multiplexed channels are directed to inputs ofthe first optical delay 24, the second optical delay 25, and the thirdoptical delay 26, respectively. A first output from the first opticaltap 22 is coupled to a first input of the clock recovery circuit 50. Asecond output from the first optical tap 22, and outputs from the first,second, and third selectively changeable optical delays 24, 25, and 26are multiplexed in the wavelength division multiplexer (WDM) 30 togenerate an output signal comprising N phase synchronized wavelengthdivision multiplexed channels. The output signal from the WDM 30 iscoupled to an input of the second optical tap 32.

[0015] A first output of the second optical tap 32 is coupled to aninput of the phase controller 40 to divert a small amount (e.g., 2%) ofthe output signal from the WDM 30 to the phase controller 40 as an errorcontrol signal. Separate N−1 outputs from the phase controller 40 arecoupled to separate ones of the first, second, and third selectivelychangeable optical delays 24, 25, and 26. The phase controller 40 isresponsive to the error control signal received from the second opticaltap 32 to generate separate delay control signals to each of the first,second, and third selectively changeable optical delays 24, 25, and 26.A second output of the second optical tap 32 is coupled to an input ofthe optical isolator 34, and an output of the optical isolator 34 iscoupled to the NOLM 60 via a first port of the 50:50 optical coupler 70.

[0016] A second port of the 50:50 optical coupler 70 is coupled to aninput of the Erbium-doped fiber amplifier (EDFA) 42 which generates anamplified output signal that is coupled to an input of the third opticaltap 44. The third optical tap 44 directs a small portion (e.g., 2%) ofthe output signal from the EDFA 42 to the photodetector 46, and theremaining portion of the output signal from the EDFA 42 as an amplifiedand synchronized N wavelength division multiplexed optical output signalfrom the MARS 10 on optical fiber 48. The photodetector 46 converts thereceived portion of the N wavelength division multiplexed optical outputsignal into a corresponding electrical control signal which is coupledto an second input of the clock recovery circuit 50. An output of theclock recovery circuit 50 is coupled to a control input of the laser 52.The laser 52 is preferably a continuous wave laser which has itsfrequency and phase controlled by a control signal from the clockrecovery circuit 50. The output from the laser 52 is coupled to the WDM76 via the optional polarization controller (POLAR. CONT.) 74 of theNOLM 60.

[0017] In an operation of the MARS 10, the input optical signalcomprising N different wavelength division multiplexed channels frominput 18 is provided as an input to the WDD 20, where the N channels areseparated and directed to the first optical tap 22, and the first,second, and third selectively changeable optical delays 24, 25, and 26,respectively. In the first, second, and third delays 24, 25, and 26,control signals generated in the phase controller 40 cause the phase ofeach of the second-to-N channels of the input signal to be synchronizedto the phase of the reference channel (CH.1) before being multiplexedand recombined in the WDM 30. An error control signal from the secondoptical tap 32 is used by the phase controller 40 to determine any phasedifferences between each of the second-to-N channels and the firstchannel. If any phase differences are detected, the phase controller 40generates appropriate control signals to the first, second, and thirdselectively changeable optical delays 24, 25, and 26 to cause the phasesof all of the N channels to be synchronized. An exemplary arrangement ofthe phase controller 40 will be discussed hereinafter in relation toFIG. 2.

[0018] The synchronized and recombined N channels from the WDM 30 arecoupled to the 50:50 optical coupler 70 of the NOLM 60 via the secondoptical tap 32 and the isolator 34. The isolator 34 is used to block anysignals from the NOLM 70 from propagating back towards the input 18 ofthe MARS 10.

[0019] In the NOLM 60, the synchronized and recombined N channel inputsignal is split into two parts by the 50:50 optical coupler 70. A firstpart of the split signal propagates in a clockwise (CW) direction in thenonlinear optical loop 71, while the second part of the split signalpropagates in a counterclockwise (CCW) direction in the nonlinearoptical loop 71. Both the first part (CW) and second part (CCW) of theinput signal propagate through physically the same path in the nonlinearoptical loop 71. Without the introduction of the clock signal from thelaser 52, the two parts of the input signal will still be in phase whenthey reach the 50:50 optical coupler 70 after traversing the loop 71.Under such condition, the first and second parts would be switched outof the 50:50 combiner 70 back towards the input 18 of the MARS 10. Theisolator 34, in turn, would block any signal from the 50:50 combiner 70from propagating back towards the input 18 of the MARS 10.

[0020] However, a phase shift (e.g., D or 180 degrees) can be introducedinto the first (CW) propagating signal when the local controlled laser52 is coupled into the NOLM 60 via the WDM 76 and co-propagates with thefirst (CW) signal. This is due to a cross-phase modulation occurringbetween the control laser 52 and the first (CW) propagating signal. Moreparticularly, the clock signal from the laser 52 cause a phasemodulation (shifts the phase) of the first CW signal. The amount ofphase modulation or shifting of the phase of the first CW signal isdependent on the power of clock signal from the laser 52. Thedispersion-shifted high nonlinearity optical fiber coil 80 inside theloop 71 is used to increase the normal nonlinear phase shift. Thisenhancing of the phase shift or phase modulation of the first CW signalpermits a decreasing of the power needed of the control laser 52 tocause a same amount of phase shift. Since the wavelength of the opticalsignal from the control laser 52 is different from the wavelengths ofthe N channel signals propagating the CW direction, a 2×1 WDM 76 is usedto couple the signal from laser 52 into the loop 71. The filter 82 isarranged to pass the N channel signals propagating in the CW and CCWdirections and block any of the signal of the control laser 52 fromreaching the 50:50 optical coupler 70.

[0021] When the first CW signal and the second CW signal are 180 degreesout of phase on returning to the 50:50 coupler 70, the two signals fromthe loop 71 are not switched out towards the input 18 of the MARS 10.Instead, the two signals from the loop 71 are switched out of the otherport of the 50:50 optical combiner 70 and coupled to an input of theErbium-doped fiber amplifier (EDFA) 42. If some small portion of the twosignals from the loop 71 is switched out of the 50:50 coupler 70 towardsthe input 18 of the MARS 10 because the two signals are not exactly 180degrees out of phase, the isolator 34 blocks such signal from reachingthe input 18 of the MARS 10. The EDFA 42 amplifies the output signalfrom the 50:50 optical combiner 70 by a predetermined amount andgenerates an output signal that is coupled to an input of the thirdoptical tap 44. The third optical tap 44 diverts a small portion (e.g.,2%) of the amplified optical signal from the EDFA 42 to thephotodetector 46, and the remaining amplified optical signal as theoutput from the MARS 10 for transmission along an output optical fiber48. The photodetector 46 converts the optical signal received from thethird optical tap 44 into a corresponding electrical control signal tothe clock recovery circuit 50.

[0022] The clock recovery circuit 50 is responsive to the portion of thechannel 1 signal obtained from the first optical tap 22 to generate anoutput clock signal that has a same frequency (bit rate) as the pulsesof the channel 1 signal. The electrical control signal from thephotodetector 46 is used by the clock recovery circuit 50 to adjust thephase of the output clock signal so as to match the phase of the Nsynchronized wavelength division multiplexed signal obtained from theoutput of the MARS 10. More particularly, the phase of the clockgenerated by the clock recovery circuit 50 is adjusted in such a way bythe electrical control signal from the photodetector 46 that the outputclock signal from the clock recovery circuit 50 is at a maximum value.Such maximum value of the output clock signal corresponds to an in-phasecondition between the control laser 52 and the synchronized andamplified N division multiplexed channel signals at the output of theMARS 10. The output clock signal from the clock recovery circuit 50 isused to directly drive the laser 52, which is then used to switch outthe synchronized clock and N channel signals from the NOLM 60 to theEDFA 42. In this manner, the laser output signal is in phase with thefirst CW N channel signal propagating in the loop 71 of the NOLM 60.

[0023] The polarization controllers 72 and 74 are optional devices andare used only if switching of the synchronized N division multiplexedchannels out of the NOLM 60 is dependent on polarization states of thesignals. When the NOLM 60 is polarization dependent, then thepolarization controllers 72 and 74 are used to optimize the extinctionratio and the efficiency of the nonlinear transmissions in the loop 71.More particularly, when a field in the loop 71 of the NOLM 60 producessome birefringence (double refraction) therein, the polarizationcontrollers 72 and 74 are needed to properly optically bias theswitching of the NOLM 60. Since the polarization state within the loop71 does not depend on the conditions of the input signal, once theoptical bias within the NOLM 60 is properly adjusted, the bias and theloop 71 is stabilized and the bias remains fixed. Because of suchoptical biasing to control the polarization state in the loop 71, theefficiency of the nonlinear transmissions from the loop to the EDFA 42are improved. However, when the design of the NOLM 60 is madepolarization insensitive, then the optional polarization controllers 72and 74 are not required.

[0024] In accordance with the present invention, all output signals fromthe MARS 10 are retimed by a retiming arrangement comprising the clockrecovery circuit 50 and the laser 52 to have a same timing as the clockgenerated in the clock recovery circuit 50, and thereby cause any timingjitter in the input signal to the MARS 10 to be corrected. Stillfurther, the pulse amplitudes of all of the N channel signals arerestored to a predetermined level by using the EDFA 42 at the output ofthe NOLM 60. Additionally, since the switching timing window isdetermined by the pulses of the control laser 52, the switched outpulses will have a same pulse shape as the pulse of the control laser52. Therefore, the output pulses from the MARS 10 are reshaped accordingto the pulse shape of the control laser 52. By a proper design of theNOLM 60 and the waveform of the control laser 52, the output pulse shapefrom the MARS 10 can be adjusted to optimize the system performance ofthe MARS 10.

[0025] Other advantages can be obtained from the design of the MARS 10.A first advantage is that any noise in each channel is reduced by theNOLM 60, which is a well known property of a NOLM. More particularly,the NOLM 60 operates as a timing gate, where it opens periodically attimes when the MARKS (or 1's) of each channel arrive, and closes inbetween. Therefore, any noise between MARKS will be blocked out.

[0026] A second advantage is that in order to optimize the systemperformance, there exists an optimal frequency chirp for a best Qfactor, or an equivalent bit error rate (BER). In the MARS 10, it ispossible to control the chirp of the output signal from the MARS 10 byadjusting the power level of the control laser 52. More particularly,modifying the switching power of the laser 52 is strictly done at thelaser 52 and not within the loop 71 since the loop 71 is a passivedevice. If the control laser 52 is stronger than required, it willintroduce a larger phase shift between the two oppositely propagating Nchannel signals. Any phase difference between the two oppositelypropagating N channel signals is proportional to the poser of the laser52. The higher the power of the laser 52, the larger the phasedifference that is introduced. Two effects occur—first, there is anintense demodulation because of the interference effect, and second,there is also a phase factor which chirps the pulses. By controlling thelaser power, the amount of chirp and the transmission from the NOLM 60will be controlled. Chirp indicates that the frequency across a pulse isnot uniform because the frequency changes over time across the pulse.For example, when a pulse is launched through a chromatic dispersantelement such as an optical fiber, the pulse will become chirped becausethe lower frequency portion moves faster when compared to the higherfrequency portion. Therefore, the leading edge of the pulse has a lowerfrequency than the trailing edge of the pulse and is referred to as a“chirp”.

[0027] A third advantage is that (a) the input signal to the MARS 10 caneither be a NonReturn to Zero (NRZ) or Return to Zero (RZ) format, or(b) the output signal format from the MARS 10 can be a RZ format orquasi RZ format. Since the RZ format provides better performance thanthe NRZ format for OC-192 systems and beyond, the MARS 10 is able tooptimize an existing NRZ transmission performance by modifying the pulseformat to a quasi NRZ. The built-in chirp control function available inthe MARS 10 is also very useful in dealing with high chromaticdispersion. Still further, the MARS 10 is also compatible with solitronsystems.

[0028] Referring now to FIG. 2, there is shown a block diagram of anexemplary arrangement of a block diagram of a phase controller 40 (shownwithin a dashed line rectangle) for use in the multi-wavelength,all-optical, regenerator system (MARS) of FIG. 1 in accordance with thepresent invention. The phase controller 40 comprises an opticalwavelength division demultiplexer (WDD) 100, a power splitter 102, aplurality of N−1 optical combiners (of which only first, second, andthird optical combiners 104A, 104B, and 104N, respectively, are shown),a plurality of N−1 photodetectors (of which only first, second, andthird photodetectors 107A, 107B, and 107N, respectively, are shown), anda plurality of N−1 bandpass filters (BPF) (of which only first, second,and third bandpass filters 110A, 110B, and 110N, respectively, areshown). For purposes of discussion hereinafter, it is assumed that N=4.An N synchronized wavelength division multiplexed channel signal fromthe second optical tap 32 of the MARS 10 of FIG. 1 is provided as aninput signal to the WDD 100. The WDD 20 is a wavelength divisiondemultiplexer which separates the N channels and directs the N differentwavelength division multiplexed channels along N separate output paths.A first channel (CH.1), which is hereinafter considered as a referencechannel, is directed along a first path to an input of the powersplitter 102. The power splitter 102 divides the first channel signalinto N−1 parts which are coupled to first inputs of separate ones of theoptical combiners 104A, 104B, and 104N. The second channel (CH.2) signalfrom the WDD 100 is coupled to a second input of the optical combiner104A, the third channel (CH.3) from the WDD 100 is coupled to a secondinput of the optical combiner 104B, and the Nth channel (CH.N) from theWDD 100 is coupled to a second input of the optical combiner 104N.Outputs from the optical combiners 104A, 104B, and 104N are coupled toinputs of the photodetectors 107A, 107B, and 107N, respectively. Outputsfrom the photodetectors 107A, 107B, and 107N are coupled to inputs ofthe BPFs 110A, 110B, and 110N, respectively. The outputs from the BPFs110A, 110B, and 110N are provided as delay control signals to the first,second, and third optical delays 22, 23, and 24, respectively, of theMARS 10 of FIG. 1.

[0029] In operation, the N synchronized wavelength division multiplexedchannel signals from the second optical tap 32 of the MARS 10 of FIG. 1are demultiplexed in the WDD 100. The optical combiner 104A combines thesecond channel (CH.2) signal from the WDD 100 with the part of thereference channel (CH.1) signal provided by the power splitter 102. Thephotodetector 107A converts the optical signal from the optical combiner104A into a corresponding electrical signal which is coupled to the BPF110A. An output error control signal from the BPF 110A is proportionalto a phase error between the demultiplexed channel 2 signal and thereference channel (CH. 1). Each of the combinations of the opticalcombiner 104B, photodetector 107B, and BPF 110B for the third channel,and the optical combiner 104N, photodetector 107N, and BPF 110N for theNth channel operate in the same manner as described hereinabove for thecombination of the optical combiner 104A, photodetector 107A, and BPF110A for the second channel. The error control signals from the BPFs110A, 110B, and 110N are simultaneously fed back to optical delays 22,23, and 24, respectively, to synchronize the phases of all of the Nchannels prior to being multiplexed in the WDM 30 (shown in FIG. 1).

[0030] It is to be appreciated and understood that the specificembodiments of the present invention described hereinabove are merelyillustrative of the general principles of the invention. Variousmodifications may be made by those skilled in the art which areconsistent with the principles set forth.

What is claimed is:
 1. An optical regenerator for use in an opticaltransmission system comprising: an optical N wavelength divisionmultiplexed channel signal synchronizing arrangement responsive to thereception of an optical input signal to the regenerator comprising Nwavelength division multiplexed channel signals for synchronizing thephase of all of the N channel signals, and for generating an opticaloutput signal comprising N synchronized wavelength division multiplexedchannel signals, where N·2; an optical pulse retiming and reshapingarrangement comprising a nonlinear optical loop mirror (NOLM)arrangement which is responsive to the optical output signal from thesignal synchronizing arrangement for retiming and reshaping opticalpulses of the optical synchronized N wavelength division multiplexedchannel signals, and for generating a corresponding optical NOLM outputsignal; and an Erbium-doped fiber amplifier (EDFA) responsive to theoptical NOLM output signal for amplifying the retimed and reshapeoptical NOLM output signal, and for generating an amplified andphase-synchronized optical N wavelength division multiplexed channeloutput signal for transmission as an output signal from the opticalregenerator.
 2. The optical regenerator of claim 1 wherein the optical Nwavelength division multiplexed channel signal synchronizing arrangementcomprises: an optical wavelength division demultiplexer (WDD) responsiveto the optical input signal to the regenerator for directing the Nwavelength division multiplexed channels onto separate N output paths; aplurality of N−1 selectively changeable optical delays coupled inpredetermined N−1 output paths of the WDD, each optical delay beingresponsive to a delay control signal for selectively adjusting the phaseof an associated one of the N−1 optical channel signals so that all ofthe N−1 optical channel signals are synchronized in phase with an Nthoptical channel signal; an optical wavelength division multiplexer (WDM)for multiplexing the Nth optical channel signal and the N−1 opticalchannel signals from the plurality of N−1 optical delays to generate asynchronized N wavelength division multiplexed channel optical outputsignal; and a phase controller responsive to a portion of the opticaloutput signal from the WDM for detecting a phase difference between theNth channel signal and each of the N−1 delayed channel signals, and forgenerating a separate delay control signal to each of the optical delaysfor synchronizing the phase of each of the N−1 delayed optical channelsignals to the phase of the Nth optical channel signal.
 3. The opticalregenerator of claim 2 wherein the phase controller comprises: awavelength division demultipexer (WDD) responsive to a portion of theoptical output signal from the WDM for directing the N channels ontoseparate N output paths; a power splitter for splitting the power of afirst one of the synchronized N wavelength division multiplexed opticalchannel signals in a first output path from the WDD into N−1 parts; aplurality of N−1 optical combiners, each optical combiner combining aseparate one of the N−1 parts of the first one of the optical channelsignals from the power splitter and a separate one of second-to-Noptical channel signals in a second-to-N output path from the WDD forgenerating an optical output signal corresponding to the combinedassociated channel signals; and a plurality of N−1 filters, each filterbeing coupled to receive a separate one of the optical output signalsfrom the plurality of N−1 combiners, and to generate a phase errorcontrol signal as an output signal to a separate predetermined one ofthe plurality of N−1 selectively changeable optical delays.
 4. Theoptical regenerator of claim 3 wherein the phase controller furthercomprises: a plurality of N−1 photodetectors, each photodetector beingcoupled to receive a separate one of the optical output signals from theplurality of N−1 optical combiners, and to convert said separate one ofthe optical output signals into a corresponding electrical signal; andthe plurality of N−1 filters are N−1 bandpass filters, each bandpassfilter being coupled to receive a separate one of the electrical signalsfrom the plurality of N−1 photodetectors and to generate a phase errorcontrol signal as an output signal to a separate predetermined one ofthe plurality of N−1 selectively changeable optical delays.
 5. Theoptical regenerator of claim 1 wherein the optical pulse retiming andreshaping arrangement further comprises: a clock recovery circuitresponsive to a predetermined one of the N channel signals propagatingin the optical N wavelength division multiplexed channel signalsynchronizing arrangement for generating a clock output signal having afrequency corresponding to the bit rate frequency of the predeterminedone of the N channel signals, and being responsive to the output signalfrom the EDFA for synchronizing the phase of the clock output signal tothe phase of the output signal from the EDFA; and a laser responsive tothe clock output signal from the clock recovery circuit for generating acorresponding optical clock output signal which is coupled into theNOLM.
 6. The optical regenerator of claim 5 wherein the NOLM comprises:an optical fiber loop wherein at least a portion thereof comprises ahigh nonlinearity optical fiber section; an optical coupler forreceiving the optical synchronized N wavelength division multiplexedchannel output signal from the signal synchronizing arrangement at afirst port thereof, and for directing first and second halves of thereceived synchronized N channel output signal for propagation in firstand second opposite directions, respectively, in the optical fiber loop;and a wavelength division multiplexer (WDM) for multiplexing the opticalclock output signal from the laser with the first half of the opticalsynchronized N wavelength division multiplexed channel output signalfrom the signal synchronizing arrangement propagating in the firstdirection in the optical fiber loop in order to shift the phase of thefirst half of the optical synchronized N channel signal by apredetermined amount and cause the first and second halves of thesynchronized N channel signals to be switched out of a second port ofthe WDM to the EDFA.
 7. The optical regenerator of claim 6 wherein theNOLM further comprises a filter for blocking any of the optical clockoutput signal from the laser from propagating to the optical coupler ofthe NOLM after the phase of the first half signal has been shifted by apredetermined amount.
 8. The optical regenerator of claim 6 wherein theNOLM further comprises: a first polarization controller coupled betweenthe laser and the WDM; and a second polarization controller coupledbetween the optical coupler and the WDM of the NOLM, wherein the firstand second polarization controllers provide a proper optical biasing ofthe optical fiber loop for stabilizing the NOLM when the optical fiberloop is not polarization independent.
 9. The optical regenerator ofclaim 1 wherein the NOLM comprises: an optical fiber loop wherein atleast a portion thereof comprises a high nonlinearity fiber; an opticalcoupler for receiving the optical synchronized N wavelength divisionmultiplexed channel output signal from the signal synchronizingarrangement at a first port thereof, and for directing first and secondhalves of the received synchronized N channel output signal forpropagation in first and second opposite directions, respectively, inthe optical fiber loop; and a wavelength division multiplexer (WDM) formultiplexing the optical clock output signal from the laser with thefirst half of the optical synchronized N wavelength division multiplexedchannel output signal from the signal synchronizing arrangementpropagating in the first direction in the optical fiber loop in order toshift the phase of the first half of the optical synchronized N channelsignal by a predetermined amount and cause the first and second halvesof the synchronized N channel signals to be switched out of a secondport of the WDM to the EDFA.
 10. The optical regenerator of claim 9wherein the NOLM further comprises a filter for blocking any of theoptical clock output signal from the laser from returning to the opticalcoupler of the NOLM after the phase of the first half signal has beenshifted by a predetermined amount.
 11. The optical regenerator of claim9 wherein the NOLM further comprises: a first polarization controllercoupled between the laser and the WDM; and a second polarizationcontroller coupled between the coupler and the WDM of the NOLM, whereinthe first and second polarization controllers provide a proper opticalbiasing of the optical fiber loop for stabilizing the NOLM when theoptical fiber loop is not polarization independent.
 12. An opticalregenerator for use in an optical transmission system comprising: anoptical wavelength division demultiplexer (WDD) responsive to an opticalinput signal to the regenerator comprising N wavelength divisionmultiplexed channels for directing the N channels onto separate N outputpaths, where N·2; a plurality of N−1 selectively changeable opticaldelays, each optical delay being coupled in a separate predetermined oneof N−1 output paths of the WDD, and being responsive to a delay controlsignal for selectively adjusting the phase of the channel signal in anassociated one of an N−1 output paths so that all N−1 optical channelsignals are synchronized in phase with an Nth optical channel signalpropagating in an output path not including an optical delay; an opticalwavelength division multiplexer (WDM) for multiplexing the Nth opticalchannel signal and the N−1 optical channel signals from the plurality ofN−1 optical delays into a synchronized N wavelength division multiplexedchannel optical output signal; a phase controller responsive to aportion of the optical output signal from the WDM for detecting a phasedifference between the Nth channel signal and each of the remaining N−1delayed channel signals, and for generating a separate delay controlsignal to each of the optical delays for synchronizing the phase of eachof the N−1 delayed optical channel signals to the phase of the Nthoptical channel signal; an optical pulse retiming and reshapingarrangement comprising a nonlinear optical loop mirror (NOLM)arrangement which is responsive to the optical synchronized N wavelengthdivision multiplexed channel output signal from the WDM for retiming andreshaping optical pulses of the optical synchronized N wavelengthdivision multiplexed channels and for generating a corresponding opticalNOLM output signal; and an Erbium-doped fiber amplifier (EDFA)responsive to the optical NOLM output signal for amplifying NOLM outputsignal, and for generating an amplified and phase-synchronized optical Nwavelength division multiplexed channel output signal for transmissionas an output signal from the optical regenerator.
 13. The opticalregenerator of claim 12 wherein the phase controller comprises: awavelength division demultipexer (WDD) responsive to a portion of theoptical output signal from the WDM for directing the N channels ontoseparate N output paths; a power splitter for splitting the power of afirst one of the synchronized N wavelength division multiplexed opticalchannel signals into N−1 parts; a plurality of N−1 optical combiners,each combiner combining a separate one of the N−1 parts of the first oneof the synchronized N wavelength division multiplexed optical channelsignals from the power splitter for generating an optical output signalcorresponding to the combined associated channel signals; and aplurality of N−1 filters, each filter being coupled to receive aseparate one of the output signals from the plurality of N−1 combinersand to generate a phase error control signal as an output signal to aseparate predetermined one of the plurality of N−1 selectivelychangeable optical delays.
 14. The optical regenerator of claim 13wherein the phase controller further comprises: a plurality of N−1photodetectors, each photodetector being coupled to receive a separateone of the optical output signals from the plurality of N−1 opticalcombiners and to convert said separate one of the optical output signalinto a corresponding electrical signal; and the plurality of N−1 filtersare N−1 bandpass filters, each bandpass filter being coupled to receivea separate one of the electrical signals from the plurality of N−1photodetectors and to generate a phase error control signal as an outputsignal to a separate predetermined one of the plurality of N−1selectively changeable optical delays.
 15. The optical regenerator ofclaim 12 wherein the optical pulse retiming and reshaping arrangementfurther comprises: a clock recovery circuit responsive to thepredetermined Nth optical channel signal in the Nth output path from theWDD for generating a clock output signal having a frequencycorresponding to the bit rate frequency of the predetermined Nth opticalchannel signal, and being responsive to the output signal from the EDFAfor synchronizing the phase of the clock output signal to the phase ofthe amplified and phase-synchronized optical N wavelength divisionmultiplexed channel in the output signal from the EDFA; and a laserresponsive to the clock output signal from the clock recovery circuitfor generating a corresponding optical clock output signal which iscoupled into the NOLM.
 16. The optical regenerator of claim 15 whereinthe NOLM comprises: an optical fiber loop wherein at least a portionthereof comprises a high nonlinearity fiber section; an optical couplerfor receiving the optical synchronized N wavelength division multiplexedchannel output signal from the WDM at a first port thereof, and fordirecting first and second halves of the received synchronized Nwavelength division multiplexed channel output signal for propagation infirst and second opposite directions, respectively, in the optical fiberloop; and a wavelength division multiplexer (WDM) for multiplexing theoptical clock output signal from the laser with the first half of theoptical N channel signal propagating in the first direction in theoptical fiber loop in order to shift the phase of the first half opticalN channel signal by a predetermined amount and cause the first andsecond halves of the N channel signals to be switched out of a secondport of the WDM to the EDFA.
 17. The optical regenerator of claim 16wherein the NOLM further comprises a filter for blocking any of theoptical clock output signal from the laser from returning to the opticalcoupler after phase of the first half signal in the optical fiber loophas been shifted by a predetermined amount.
 18. The optical regeneratorof claim 16 wherein the NOLM further comprises: a first polarizationcontroller coupled between the laser and the WDM; and a secondpolarization controller coupled between the coupler and the WDM, whereinthe first and second polarization controllers provide a proper opticalbiasing of the optical fiber loop for stabilizing the NOLM when theoptical fiber loop is not polarization independent.
 19. The opticalregenerator of claim 12 wherein the NOLM comprises: an optical fiberloop wherein at least a portion thereof comprises a high nonlinearityfiber; an optical coupler for receiving the optical synchronized Nwavelength division multiplexed channel output signal from the WDM at afirst port thereof, and for directing first and second halves of thereceived synchronized N channel output signal for propagation in firstand second opposite directions, respectively, in the optical fiber loop;and a wavelength division multiplexer (WDM) for multiplexing an opticalclock output signal with the first half of the optical N channel signalfrom the WDM that is propagating in the first direction in the opticalfiber loop in order to shift the phase of the first half signal by apredetermined amount and cause the first and second halves of thesynchronized N channel signals to be switched out of a second port ofthe optical coupler of the NOLM to the EDFA.
 20. The optical regeneratorof claim 19 wherein the NOLM further comprises a filter for blocking anyof the optical clock signal from returning to optical coupler of theNOLM after the phase of the first half signal has been shifted by apredetermined amount in the optical fiber loop.
 21. The opticalregenerator of claim 19 wherein the NOLM further comprises: a firstpolarization controller coupled between a source of the optical clocksignal and the WDM; and a second polarization controller coupled betweenthe coupler and the WDM, wherein the first and second polarizationcontrollers provide a proper optical biasing of the optical fiber loopwhen the optical fiber loop is not polarization independent.
 22. Anoptical regenerator for use in an optical transmission systemcomprising: an optical N wavelength division multiplexed channel signalsynchronizing arrangement responsive to the reception of an opticalinput signal to the regenerator comprising N wavelength divisionmultiplexed channels for synchronizing the phase of all of the N channelsignals, and for generating an optical synchronized N wavelengthdivision multiplexed channel output signal, where N·2; an optical pulseretiming and reshaping arrangement comprising: a nonlinear optical loopmirror (NOLM) arrangement which is responsive to the optical outputsignal from the signal synchronizing arrangement for retiming andreshaping optical pulses of the N channel signals, and for generating acorresponding optical NOLM output signal; a clock recovery circuitresponsive to a predetermined one of the N channel signals propagatingin the optical synchronizing arrangement for generating a clock outputsignal having a frequency corresponding to the bit rate frequency of thepredetermined one of the N channel signals, and being responsive to anoutput signal from the optical regenerator comprising Nphase-synchronized optical channel signals for synchronizing the phaseof the clock output signal to the phase of the N channel signals in theoutput signal from the optical regenerator; and a laser responsive tothe clock output signal from the clock recovery circuit for generating acorresponding optical clock output signal which is coupled into theNOLM; and an Erbium-doped fiber amplifier (EDFA) for amplifying theoptical NOLM output signal, and for generating an amplified andphase-synchronized optical N wavelength division multiplexed channeloutput signal for transmission as the output signal from the opticalregenerator.
 23. The optical regenerator of claim 22 wherein the NOLMcomprises: an optical fiber loop wherein at least a portion thereofcomprises a high nonlinearity optical fiber section; an optical couplerfor receiving the optical synchronized N wavelength division multiplexedchannel output signal from the signal synchronizing arrangement at afirst port thereof, and for directing first and second halves of thereceived synchronized N channel output signal for propagation in firstand second opposite directions, respectively, in the optical fiber loop;and a wavelength division multiplexer (WDM) for multiplexing the opticalclock output signal from the laser with the first half of the opticalsynchronized N wavelength division multiplexed channel output signalfrom the signal synchronizing arrangement propagating in the firstdirection in the optical fiber loop in order to shift the phase of thefirst half signal by a predetermined amount and cause the first andsecond halves of the N channel signals to be switched out of a secondport of the WDM to the EDFA.
 24. The optical regenerator of claim 23wherein the NOLM further comprises a filter for blocking any of theoptical clock output signal from the laser returning to optical couplerafter the phase of the first half signal has been shifted by apredetermined amount in the optical fiber loop.
 25. The opticalregenerator of claim 23 wherein the NOLM further comprises: a firstpolarization controller coupled between the laser and the WDM of theNOLM; and a second polarization controller coupled between the couplerand the WDM of the NOLM, wherein the first and second polarizationcontrollers provide a proper optical biasing of the optical fiber loopfor stabilizing the NOLM when the optical fiber loop is not polarizationindependent.
 26. The optical regenerator of claim 22 wherein the opticalN wavelength division multiplexed channel signal synchronizingarrangement comprises: an optical wavelength division demultiplexer(WDD) responsive to the optical input signal to the regenerator fordirecting the N wavelength division multiplexed channels onto separate Noutput paths; a plurality of N−1 selectively changeable optical delayscoupled in predetermined N−1 output paths of the WDD, each optical delaybeing responsive to a delay control signal for selectively adjusting thephase of an associated one of N−1 optical channel signals so that all ofthe N−1 optical channel signals are synchronized in phase with an Nthoptical channel signal not associated with an optical delay; an opticalwavelength division multiplexer (WDM) for multiplexing the Nth opticalchannel signal and the N−1 optical channel signals from the plurality ofN−1 optical delays into a synchronized N wavelength division multiplexedchannel optical output signal; and a phase controller responsive to aportion of the synchronized N wavelength division multiplexed channeloutput signal from the WDM for detecting a phase difference between theNth channel signal and each of the N−1 delayed channel signals, and forgenerating a separate delay control signal to each of the optical delaysfor synchronizing the phase of each of the N−1 delayed optical channelsignals to the phase of the Nth optical channel signal.
 27. The opticalregenerator of claim 26 wherein the phase controller comprises: awavelength division demultipexer (WDD) responsive to a portion of thesynchronized N wavelength division multiplexed channel output signalfrom the WDM for directing the N channels onto separate N output paths;a power splitter for splitting the power of a first one of thesynchronized N wavelength division multiplexed optical channel signalsinto N−1 parts; a plurality of N−1 optical combiners, each opticalcombiner combining a separate one of the N−1 parts from the powersplitter and a separate one of the remaining second-to-N wavelengthdivision multiplexed optical channel signals for generating an opticaloutput signal corresponding to the combined associated channel signals;and a plurality of N−1 filters, each filter being coupled to receive aseparate one of the output signals from the plurality of N−1 combinersand to generate a phase error control signal as an output signal to aseparate predetermined one of the plurality of N−1 selectivelychangeable optical delays.
 28. The optical regenerator of claim 27wherein the phase controller further comprises: a plurality of N−1photodetectors, each photodetector being coupled to receive a separateone of the optical output signals from the plurality of N−1 opticalcombiners, and to convert said separate one of the optical output signalinto a corresponding electrical signal; and the plurality of N−1 filtersare N−1 bandpass filters, each bandpass filter being coupled to receivea separate one of the electrical signals from the plurality of N−1photodetectors and to generate a phase error control signal as an outputsignal to a separate predetermined one of the plurality of N−1selectively changeable optical delays.
 29. A method of regenerating areceived optical N wavelength division multiplexed channel signal in anoptical regenerator of an optical transmission system, where N·2,comprising the steps of: (a) concurrently and separately synchronizingthe phase of all of the N channel signals in the received optical Nwavelength division multiplexed channel signal for generating an opticaloutput signal comprising N synchronized wavelength division multiplexedchannel signals, where N·2; (b) splitting the optical output signal fromstep (a) into two halves in an optical coupler so that a first halfpropagates in a first direction around an optical fiber loop wherein atleast a portion thereof comprises a high nonlinearity optical fibersection and a second half propagates in a second opposite directionaround the optical fiber loop; (c) multiplexing an optical clock outputsignal from a laser with the first half signal propagating in the firstdirection in the optical fiber loop in step (b) in an optical wavelengthdivision multiplexer for shifting the phase of the first half signal bya predetermined amount and causing the first and second halves of the Nchannel signals to be switched out of a second port of the opticalcoupler as a NOLM output signal when they return to the entry point ofthe optical fiber loop; and (d) amplifying the optical NOLM outputsignal in an Erbium-doped fiber amplifier (EDFA) for generating anamplified and phase-synchronized optical N wavelength divisionmultiplexed channel output signal for transmission as an output signalfrom the optical regenerator.
 30. The method of claim 29 wherein inperforming step (a), performing the steps of: (e) directing the Nchannel signals of the received optical N wavelength divisionmultiplexed channel signal onto separate N output paths of an opticalwavelength division demultiplexer (WDD); (f) selectively adjusting thephase of each of N−1 optical channel signals in N−1 output paths of theWDD in separate one of a plurality of N−1 selectively changeable opticaldelays so that all of the N−1 optical channel signals are synchronizedin phase with an Nth optical channel signal not associated with anoptical delay; (g) multiplexing the Nth optical channel signal and theN−1 optical channel signals from the plurality of N−1 optical delaysinto a synchronized N wavelength division multiplexed channel opticaloutput signal in an optical wavelength division multiplexer (WDM); (h)detecting a phase difference between the Nth channel signal and each ofthe N−1 delayed channel signals in the optical output signal from step(g) in a phase controller; and (i) generating a separate delay controlsignal from the phase controller to each of the optical delays forsynchronizing the phase of each of the N−1 delayed optical channelsignals to the phase of the Nth optical channel signal.